Repairing Inconsistent XML Data with Functional Dependencies

The World Wide Web is of strategic importance as a global repository for information and a means of communicating and sharing knowledge. Its explosive growth has caused deep changes in all the aspects of human life, has been a driving force for the development of modern applications (e.g., Web portals, digital libraries, wrapper generators, etc.), and has greatly simplified the access to existing sources of information, ranging from traditional DBMS to semi-structured Web repositories. The adoption by the WWW consortium (W3C) of XML (eXtensible Markup Language) as the new standard for information exchange among Web applications has led researchers to investigate classical problems in the new environment of repositories containing large amounts of data in XML format. Great attention has also been recently devoted to the introduction of integrity constraints and the definition of normal forms for XML (Arenas & Libkin, 2003, 2004; Fan & Libkin, 2002; Vincent & Liu, 2003). XML allows a simple form of constraints to describe references obtained through ID/IDREF, but it does not actually provide a general mechanism for expressing semantic constraints like those commonly used in relational databases. The need of enriching the semantics of XML is so deep as a large amount of XML data originates in object-oriented and relational databases, where different forms of integrity constraints are used to add semantics to the collected information. This work stems from the need of enriching the semantics of XML documents. This need is attested by several new works which introduce different forms of constraints to XML documents (Arenas, Fan & Libkin, 2002, 2004; Buneman et al., 2001, 2002; Fan & Libkin, 2002; Fan & Simeon, 2000; Vincent et al., 2004; Yang, Yu & Wang, 2001). Most of them introduce a simple form of constraints such as keys and foreign keys, whereas some others attempt to extend the class of integrity constraints associated with XML documents. Obviously, reasoning about constraints in the presence of an incomplete knowledge of the data structure is rather complex so that some of these attempts are likely to be a purely theoretical exercise. In fact, their practical applicability follows the solution of non-trivial problems such as the implication and interaction among constraints which are far from being solved. In the presence of constraints on data, an XML document may result in being inconsistent; that is, it does not respect some constraint. The following example shows the case of an inconsistent XML document.